Cryptochrome 1 mediates light-dependent inclination magnetosensing in monarch butterflies

The study investigates how monarch butterflies detect and use the Earth’s magnetic field, focusing on the molecular basis of light-dependent magnetoreception. Two main models exist for animal magnetosensing: magnetite-based mechanisms and radical-pair-based mechanisms. In the radical-pair hypothesis, photoexcited molecules generate spin-correlated radical pairs whose singlet–triplet interconversion is influenced by the Earth’s magnetic field, potentially altering biochemical signaling. Cryptochromes (CRYs), light-sensitive flavoproteins, are prime candidates for such photomagnetoreceptors. While Drosophila studies demonstrated that light-sensitive type 1 CRY mediates wavelength-dependent magnetic responses and suggested that mammalian-like type 2 CRYs might also confer magnetosensitivity when overexpressed in flies, questions remain due to non-physiological magnetic field strengths and overexpression artifacts. This study aims to genetically test the roles of CRY1 and CRY2 in monarch butterflies, an organism with a documented inclination compass, under Earth-strength magnetic fields and to identify the sensory organs involved.

Prior work showed that a radical-pair chemical compass can operate at Earth-strength magnetic fields using synthetic model systems, and Arabidopsis CRY1 forms magnetically sensitive radical pairs via its FAD cofactor. In animals, type 1 CRYs (Drosophila-like) are light-sensitive and entrain circadian clocks; type 2 CRYs (mammalian-like) are generally light-insensitive repressors. Drosophila genetic and behavioral studies provided the first in vivo evidence that type 1 CRY mediates UV-A/blue light-dependent magnetoreception, with subsequent reports that monarch and human CRY2s could rescue magnetoresponses in fly CRY mutants under strong, non-physiological fields, raising debate about CRY2’s role given its poor FAD-binding features. Previous monarch studies indicated an inclination compass that depends on short-wavelength light, and antennae had been implicated as magnetosensory organs, but a genetic dissection of CRY contributions and organ-level requirements under Earth-strength fields in monarchs was lacking.

• Behavioral assay: Developed an individual-level assay measuring magnetic hyperactivity (MH) as an increase in wingbeat power in response to reversal of magnetic inclination. Monarchs were tethered in the center of a flight simulator surrounded by a custom three-axis Helmholtz coil system capable of manipulating declination, inclination, and intensity at Earth-strength levels. An infrared beam recorded wingbeats in 10 s bins.
• Light conditions: Illumination from above with full spectrum light (~350–800 nm; ~4.35 × 10^5 photons s−1 cm−2) and filtered bands including UV-A/blue (~380–430 nm) and cyan/green (~480–580 nm). Monarchs were acclimated in darkness ≥10 min before trials.
• Magnetic protocol: Each individual experienced 2 min of constant ambient magnetic inclination (AMI; control), then after 5 min, another 2 min in which the inclination was reversed for 10 s starting 20 s after AMI onset (RAMI), then returned to AMI. Responses were quantified as increased wingbeat counts immediately following inclination reversals.
• Subjects: Both wild-caught fall migrants and laboratory-raised monarchs were tested to validate assay robustness.
• Genetics: CRISPR/Cas9 was used to generate a dpCry1 mutant carrying a 2-bp deletion in exon 4, introducing a premature stop codon and truncating the protein, removing the C-terminal domain containing the Trp tetrad implicated in electron transfer and radical-pair formation. The mutant showed ~90% reduction in dpCry1 mRNA consistent with nonsense-mediated decay; a truncated FLAG-tagged dpCRY1 protein was characterized. A dpCry2 null line was also generated and tested.
• Organ manipulation: To localize magnetosensory organs, light input to antennae and/or compound eyes was blocked with non-toxic black paint; clear paint served as control. Combinations included antennae-only, eyes-only, and unilateral antenna plus eye.
• Expression analyses: dpCry1 transcript levels assessed by qPCR and protein by Western blot in antennae, compound eye photoreceptors, and optic lobe of adult monarchs.
• Rearing and housing: Laboratory-raised monarchs from eggs on milkweed; larvae reared on semi-artificial diet; adults maintained under controlled LD cycles, temperature, and humidity. Wild-caught fall migrants were collected in Texas and housed indoors with season-mimicking LD cycle.
• Statistics: Nonparametric Mann–Whitney U tests were applied per 10 s bin due to non-normal distributions/heteroscedasticity; significance at p < 0.05. Field parameters were monitored with a magnetometer; potential heating by coils and temperature were controlled.

• Behavioral validation: Monarchs exhibited a robust increase in wingbeat power (magnetic hyperactivity, MH) following reversals of magnetic inclination (AMI↔RAMI) under full-spectrum light at Earth-strength field intensities. No hyperactivity occurred under constant AMI. Wild-caught and lab-raised monarchs responded similarly (n = 32 each; no significant differences across bins; two-tailed Mann–Whitney U tests). MH increases occurred within ~15 s after inclination reversal (p < 0.001 vs. constant AMI).
• Wavelength dependence and CRY1 requirement: Under full-spectrum and UV-A/blue light, dpCry1 mutants showed significantly reduced or absent MH compared to wild type, indicating CRY1 is necessary for light-dependent inclination magnetoreception. In Fig. 2, significant genotype effects appeared in specific 10 s bins (e.g., full spectrum: p = 0.003 and 7.000E–6 for select bins; UV-A/blue: p = 0.038 and 2.000E–6 for select bins). Under cyan/green light (480–580 nm), MH was not observed, consistent with short-wavelength dependence.
• CRY2 dispensable: dpCry2−/− monarchs displayed MH indistinguishable from dpCry2+/+ across lighting conditions (full spectrum, darkness, UV-blue, UV-green), with no significant differences in any time bin (n = 30 WT, n = 25 mutant), indicating CRY2 is not required for light-dependent inclination magnetosensing at Earth-strength magnetic fields.
• Magnetosensory organs: Painting antennae black significantly reduced MH versus clear-painted controls under full-spectrum and UV-A/blue light (p < 0.03 in key bins). Painting eyes black produced a similar reduction. Combined manipulations (one antenna and one eye blocked) also impaired responses. dpCry1 transcript and protein were enriched in antennae and eye photoreceptor layers relative to optic lobe, supporting these as magnetosensory sites.
• Overall: The monarch’s inclination compass is UV-A/blue light-dependent, requires CRY1, and involves antennae and compound eyes. CRY2 does not contribute under physiological conditions.

The findings provide genetic and behavioral evidence that CRY1 mediates light-dependent inclination compass sensing in monarch butterflies at Earth-strength magnetic fields, extending the role of type 1 cryptochromes in magnetoreception beyond Drosophila and addressing concerns about prior non-physiological field strengths and overexpression contexts. The assay demonstrates that inclination reversals elicit rapid, reproducible behavioral responses (MH), linking CRY1 function to detection of the vector direction of the geomagnetic field. The lack of effect of CRY2 loss-of-function challenges interpretations from Drosophila transgenic rescue studies and supports the view that only light-sensitive cryptochromes function in light-dependent animal magnetoreception in relevant cellular contexts. Organ-level manipulations localize the magnetic sense to antennae and eyes, consistent with CRY1 expression patterns. Mechanistically, results align with a radical-pair-based photoreception model relying on CRY1, though direct demonstration of radical-pair chemistry in vivo remains outstanding. The absence of responses under green/cyan light suggests either insufficient CRY1 photoactivation at longer wavelengths or potential involvement of additional photoreceptors (e.g., opsins) with different spectral sensitivities. These insights lay groundwork for dissecting the photochemistry and neural circuitry of the monarch magnetic compass and for examining interactions with other orientation systems (e.g., time-compensated sun compass).

This study establishes monarch butterflies as a genetically tractable model for light-dependent inclination magnetoreception, demonstrating that CRY1—but not CRY2—is essential for UV-A/blue light-dependent responses to geomagnetic inclination reversals, and that both antennae and compound eyes serve as magnetosensory organs. The work strengthens the case that light-sensitive cryptochromes function in animal magnetoreception under natural field strengths and links CRY1 to geomagnetic compass function. Future research should directly test radical-pair mechanisms in CRY1 within native tissues, define spectral requirements and potential roles of other photoreceptors (e.g., opsins), map the neural circuits from antennae and eyes underlying magnetic sensing, and determine how the magnetic compass integrates with or complements sun compass and potential map-based navigation during migration.

The study does not directly demonstrate radical-pair formation or CRY1 photochemistry in vivo; thus, the mechanistic basis (e.g., FAD redox states and electron-transfer pathways) remains inferential. Behavioral readouts rely on wingbeat hyperactivity rather than sustained orientation, which may not capture all aspects of compass use. Green/cyan light non-responsiveness could reflect insufficient CRY1 activation or involvement of other photoreceptors, which was not resolved. Findings are from monarchs and may not generalize to all taxa. Some methodological details (e.g., exact CRY2 mutation characterization) are summarized without full molecular elaboration in the provided text.